Fixtures of this kind are used in the field of prosthetic surgery, wherein the prosthesis, parts of a prosthesis or a prosthesis holder is/are anchored in the bone tissue of a person with the aid of one or more such fixtures.
However, the fixtures according to the present invention are intended primarily to be anchored in hollow or tubular bone and can be used, for instance, in the reconstruction of joints, for instance finger joints or hip joints. The fixtures can also be used to fasten artificial legs or other types of prostheses. However, the invention is not limited to such applications. The fixture has an anchoring portion and an application portion. The anchoring portion is screwed into a hole predrilled in the bone tissue. The hole is given a diameter which is smaller than the outer diameter of the screw thread of the anchoring portion. The anchoring portion is self-tapping and thus produces a screw thread in the wall of the predrilled hole when screwed thereinto, while removing bone fragments. Screwing of the anchoring portion into said hole is facilitated by the slots, by virtue of the bone fragments being pulled loose from the hole by the edges formed between the outer periphery of the anchoring portion and the slots. These bone fragments are conveyed to said cavity through the slots.
An example of such a fixture is described in EP 0 595 782, in which the anchoring end of the fixture is provided with four helically extending slots. This known fixture has been found highly satisfactory in many respects and has provided reliable and positive anchorage of a prosthesis fastened with such a fixture.
However, the known fixture has certain drawbacks. For example, as the fixture is rotated, the bone tissue is worked by those edges that are formed between the periphery of the anchoring portion and the trailing walls of respective slots. The edge between the radially directed slot wall and said peripheral surface generally defines a right angle. Such an edge works the bone tissue primarily by rubbing against and tearing loose bone fragments from the wall of the predrilled hole when screwing in the anchoring portion. This results in an unfavourable harsh action on the bone tissue. Moreover, the bone fragments that are so loosened are unusually large and of an irregular shape. This makes recovery and healing of the bone material that collects in the cavity of said fixture difficult to achieve.
Another drawback with the known fixture is that the radial slots cause the geometry of the anchoring portion to become unfavourable when said portion is subjected to the compressive radially acting forces created when screwing the anchoring portion into the predrilled hole. The combined effect of the rotation of the anchoring portion and of the forces that act radially inwards cause the parts situated between the slots to become distorted as a result of said compression, therewith causing the fastening portion to be deformed to a non-round shape. This reduces the strength of the anchorage. Although this deformation can be avoided by increasing the wall thickness of said portions, this increase in wall thickness will cause the anchoring portion to become stiffer and make it difficult to achieve the radial elasticity desired to achieve a strong anchorage.
The object of the present invention is to provide an improved type of fixture in which the aforedescribed drawbacks are eliminated or at least reduced. The primary object of the invention is to provide a so-called fixture with which the bone fragments will be detached favourably when screwing in the fixture.